Polymers are used for drug delivery for a variety of therapeutic purposes. Polymers have also been used for the delivery of nucleic acids (polynucleotides and oligonucleotides) to cells for therapeutic purposes that have been termed gene therapy or anti-sense therapy. One of the several methods of nucleic acid delivery to the cells is the use of DNA-polycations complexes. It was shown that cationic proteins like histones and protamines or synthetic polymers like polylysine, polyarginine, polyomithine, DEAE dextran, polybrene, and polyethylenimine were effective intracellular delivery agents while small polycations like spermine were ineffective. (Felgner, P. L. (1990) Advanced Drug Delivery Rev. 5, 163–187; Boussif, O., Lezoualch, F., Zanta, M. A., Mergny, M. D., Scherman, D., Demeneix, B., & Behr, J. P. (1995) Proc. Natl. Acad. Sci. USA 92, 7297–7301) The mechanism by which polycations facilitate uptake of DNA is not completely understood but the following are some important principles:
1) Polycations provide attachment of DNA to the target cell surface: The polymer forms a cross-bridge between the polyanionic nucleic acids and the polyanionic surfaces of the cells. As a result the main mechanism of DNA translocation to the intracellular space might be non-specific adsorptive endocytosis which may be more effective then liquid endocytosis or receptor-mediated endocytosis. Furthermore, polycations are a very convenient linker for attaching specific receptors to DNA and as result, DNA-polycation complexes can be targeted to specific cell types. (Perales, J. C., Ferkol, T. Molas, M. & Hanson, W. (1994) Eur. J. Biochem. 226, 255–266; Cotten, M., Wagner, E. & Bimstiel, M. L. (1993) Methods in Enzymology 217, 618–644; Wagner, E., Curiel, D., & Cotten, M. (1994) Advanced Drug Delivery Rev. 1 14, 113–135).
2) Polycations protect DNA in complexes against nuclease degradation (Chiou, H. C., Tangco, M. V. Levine, S. M., Robertson, D., Kormis, K., Wu, C. H., & Wu, G. Y. (1994) Nucleic Acids Res. 22, 5439–5446). This is important for both extra- and intracellular preservation of DNA. The endocytic step in the intracellular uptake of DNA-polycation complexes is suggested by results in which DNA expression is only obtained by incorporating a mild hypertonic lysis step (either glycerol or DMSO) (Lopata, M. A., D. Clevland, W., & Sollner-Webb, B. (1984) Nucleic Acids Res. 12, 5707–5717; Golub, E. I., Kim, H. & Volsky, D. J. (1989) Nucleic Acid Res. 17, 4902). Gene expression is also enabled or increased by preventing endosome acidification with NH4CI or chloroquine (Luthman, H. & Magnusson, G. (1983) Nucleic Acids Res. 11, 1295–1300). Polyethylenimine which facilitates gene expression without additional treatments probably disrupts endosomal function itself (Boussif, O., Lezoualch, F., Zanta, M. A., Mergny, M. D., Scherman, D., Demeneix, B., & Behr, J. P. (1995) Proc. Natl. Acad. Sci. USA 92, 7297–7301). Disruption of endosomal function has also been accomplished by linking to the polycation endosomal-disruptive agents such as fusion peptides or adenoviruses (Zauner, W., Blaas, D., Kuechler, E., Wagner, E., (1995) J. Virology 69, 1085–1092; Wagner, E., Plank, C., Zatloukal, K., Cotten, M., & Birnstiel, M. L. (1992) Proc. Natl. Acad. Sci. 89, 7934–7938) (Fisher, K. J., & Wilson, J. M. (1994) Biochemical J. 299, 49–58).
3) Polycations generate DNA condensation: The volume which one DNA molecule occupies in complex with polycations is drastically lower than the volume of a free DNA molecule. The size of DNA/polymer complex is critical for gene delivery in vivo. In terms of intravenous injection, DNA needs to cross the endothelial barrier and reach the parenchymal cells of interest. The largest endothelia fenestrae (holes in the endothelial barrier) occurs in the liver and have an average diameter 100 nm. The fenestrae size in other organs is much lower. The size of the DNA complexes is also important for the cellular uptake process. After binding to the target cells the DNA-polycation complex should be taken up by endocytosis. Since the endocytic vesicles have a homogenous internal diameter of about 100 nm in hepatocytes of similar size in other cell types, the DNA complexes need to be smaller than 100 nm (Geuzze, H. J., Slot, J. W., Strous, G. J., Lodish, H. F., & Schwartz, A. L. (1982) J.Cell Biol. 92, 865–870).
Condensation of DNA
A significant number of multivalent cations with widely different molecular structures have been shown to induce the condensation of DNA. These include spermidine, spermine, Co(NH3)63+, protamine, histone H1, and polylysine. (Gosule, L. C. & Schellman, J. A. (1976) Nature 259, 333–335; Chattoraj, D. K., Gosule, L. C. & Schellman, J. A. (1978) J. Mol. Biol. 121, 327–337; Had, N. V., Downing, K. H. & Balhorn, R. (1993) Biochem. Biophys. Res. Commun. 193, 1347–1354; Hsiang, M. W & Cole, R. D. (1977) Proc. Natl. Acad. Sci. USA 74, 4852–4856; Haynes, M., Garret, R. A. & Gratzer, W. B. (1970) Biochemistry 9, 4410–4416; Widom, J. & Baldwin, R. L. (1980) J. Mol. Biol. 144, 431–453.). Quantitative analysis has shown DNA condensation to be favored when 90% or more of the charges along the sugar-phosphate backbone are neutralized (Wilson, R. W. & Bloomfield, V. A. (1979) Biochemistry 18, 2192–2196). Depending upon the concentration of the DNA condensation leads to three main types of structures:
1) In extremely dilute solution (about 1 ug/ml or below), long DNA molecules can undergo a monomolecular collapse and form structures described as toroid.
2) In very dilute solution (about 10 ug/ml) microaggregates form with short or long molecules and remain in suspension. Toroids, rods and small aggregates can be seen in such solution.
3) In dilute solution (about 1 mg/ml, large aggregates are formed that sediment readily. (Sicorav, J.-L., Pelta, J., & Livolant, F (1994) Biophysical Journal 67, 1387–1392).
Toroids have been considered an attractive form for gene delivery because they have the lowest size. While the size of DNA toroids produced within single preparations has been shown to vary considerably, toroid size is unaffected by the length of DNA being condensed. DNA molecules from 400 bp to genomic length produce toroids similar in size (Bloomfield, V. A. (1991) Biopolymers 31, 1471–1481). Therefore one toroid can include from one to several DNA molecules. The kinetics of DNA collapse by polycations which resulted in toroids is very slow. For example DNA condensation by Co(NH3)6CI3 needs 2 hours at room temperature. (Arscott, P. G., Ma, C., & Bloomfield, V. A. (1995) Biopolymers 36, 345–364).
The mechanism of DNA condensation is not obvious. The electrostatic forces between unperturbed helices arise primarily from a counterion fluctuation mechanism requiring multivalent cations and plays the major role in DNA condensation. (Riemer, S. C. & Bloomfield, V. A. (1978) Biopolymers 17, 789–794; Marquet, R. & Houssier, C. (1991) J. Biomol. Struct. Dynam. 9, 159–167; Nilsson, L. G., Guldbrand, L. & Nordenskjold L. (1991) Mol. Phys. 72, 177–192). The hydration forces predominate over electrostatic forces when the DNA helices approach closer then a few water diameters (Leikin, S., Parsegian, V. A., Rau, D. C. & Rand, R. P. (1993) Ann. Rev. Phys. Chem. 44, 369–395). In case of DNA-polymeric polycation interactions, DNA condensation is a more complicated process than the case of low molecular weight polycations. Different polycationic proteins can generate toroid and rod formation with different size DNA at a ratio of positive to negative charge of 0.4 (Garciaramirez, M., & Subirana, J. A. (1994) Biopolymers 34, 285–292). It was shown by fluorescence microscopy that T4 DNA complexed with polyarginine or histone can forms two types of structures; an elongated structure with a long axis length of about 350 nm (like free DNA) and dense spherical particles. (Minagawa, K., Matsuzawa, Y., Yshikawa, K., Matsumoto, M., & Doi, M. (1991) FEBS Lett. 295, 60–67). Both forms exist simultaneously in the same solution. The reason for the co-existence of the two forms can be explained as an uneven distribution of the polycation chains among the DNA molecules. The uneven distribution generates two thermodynamically favorable conformations. (Kabanov, A. V., & Kabanov, V. A. (1995) Bioconjugate Chem. 6, 7–20).
It was also shown that the electrophoretic mobility of DNA-polycation complexes can change from negative to positive in excess of polycation. It is likely that large polycations don't completely align along DNA but form polymer loops which interact with other DNA molecules. The rapid aggregation and strong intermolecular forces between different DNA molecules may prevent the slow adjustment between helices needed to form tightly packed, orderly particles. This specification describes a new approach, that we have termed Polynucleotide Template Polymerization, for overcoming this problem of nonspecific aggregation and large DNA-polycation complex formation that occurs when polycation/DNA complexes are formed in DNA concentrations that are of practical value for polynucleotide transfer into cells and for gene or antisense therapy.